1. Field of the Invention
This invention is in the field of methods of cleaning copper surfaces so that such surfaces are readily solderable.
2. Description of the Prior Art
Copper surfaces that are to be soldered to other compatible metal surfaces, such as copper, typically need to be cleaned prior to being soldered either mechanically or chemically. Mechanical processes usually abrade the outer, or exposed, surfaces of the copper to produce a surface that is not contaminated by materials that would prevent a tin/lead solder, for example, from wetting the surfaces. When physical processes of cleaning a surface to be soldered are not practical or desirable because of the damage they cause, chemical processes for cleaning such surfaces are used. The active ingredient of most chemical processes is a dilute solution of phosphoric acid or a dilute solutioon of ammonium hydroxide, for example.
The development of integrated circuits (I.C.) semiconductor devices with ever-increasing numbers of active gates, or transistors, per device and with the delay per gate decreasing, has resulted in a requirement for better ways of interconnecting such I.C. chips into desired circuits by using conductors of minimum length to take advantage of the high-speed characteristics of such devices. One way of interconnecting such devices is to mount I.C. chips on a multilayer substrate and to interconnect the chips on a substrate by means of multiple layers of conductor runs, or conductors. The conductors are separated both horizontally and vertically by layers of a dielectric, or electrical insulator, except for the top surfaces of the top one or two of such conductor layers. Connections between conductors in different layers are made by vias, openings in the dielectric layers, which vias are then filled with an electrical conductor.
The base of many such multilayer substrates is a plate or sheet of alumina, Al.sub.2 O.sub.3. Because of the characteristics of an alumina base; namely, that it can be subjected to high temperatures without substantial damage or change of dimension, alumina bases are frequently used to build multilayer substrates. On such alumina bases, layers of thick film pastes, either conductor or dielectric pastes, are sequentially applied to create alternate layers of conductors and insulators with the necessary interconnections between conductors and devices to produce a desired electronic circuit. After a layer of conductor or dielectric paste is applied, using a conventional silkscreening printing process, the layer is fired to cause it to adhere to the base, or lower layers, and to cause the particles of the layer to fuse together. The uppermost layer or layers of conductors, particularly where the terminals of I.C. chips are to be connected into the circuit or on which the chips themselves are to be mounted, is left exposed.
As mentioned above, thick film pastes containing particles of a good electrical conductor are applied typically by a screen printing process. To minimize corrosion and to achieve good electrical conductivity, thick film pastes containing a large percentage of gold particles have heretofore been the most common type of thick film electrically conductive paste used to produce the conductor runs of such substrates. The high cost of gold has been a powerful stimuli to use less costly conductive metals in the thick film conductor pastes. Copper thick film pastes have been developed, and multilayer substrates having conductor runs principally of a fired layer of a copper thick film paste have been built. However, attempts to solder electrical leads with conventional tin/lead solders to exposed surfaces of such copper runs have encountered problems because the melted solder would not wet, or adhere to, the exposed surfaces of copper on the substrate. Because of the structure of the substrate, mechanical abrasion processes for cleaning such areas is not an acceptable solution as mechanical cleaning damages the upper surface of the substrate to the point where the substrate would be unusable for its intended purpose. Tests using the typical chemical processes using dilute solutions of phosphoric acid or of ammonium hydroxide did not result in such surfaces becoming solderable.